In the United States, given the current distribution of HIV in the population, there is better than a one in 1,000 chance of contracting HIV during an unprotected heterosexual encounter, according to the Centers for Disease Control and Prevention (CDC). In some locations, the chances are even higher. Unprotected sex is the most common way of transmitting HIV. Your chances for infection increase with each new partner. Here is a list of ways in which HIV can be transmitted:

Sexual contact
Sharing contaminated intravenous needles
Breastfeeding (mother to baby)
Infected mother to fetus during pregnancy or birth
Blood transfusions (Rare in countries where blood is screened for HIV antibodies.)

There is also a slight chance of transmission through open-mouth kissing and biting. However, there have been very few cases of HIV being transmitted through either method. In fact, the CDC has investigated only one case in which HIV infection was attributed to open-mouth kissing.

HIV does not transmit through the air or surface contact like cold and flu viruses do. HIV is a fragile virus and doesn't survive well outside the human body. This fragility makes the possibility of environmental transmission very remote. Outside of a host cell, HIV doesn't survive for very long. In laboratory studies, the CDC has shown that once the fluid (blood, sweat, tears, et cetera) containing the HIV virus dries, the risk of environmental transmission is nearly zero.

There is a lot of misinformation about how HIV can be transmitted. So, here is a list of ways in which HIV is not transmitted:

Saliva, tears and sweat - Saliva and tears contain only small amounts of HIV, and scientists haven't detected any HIV in the sweat of an infected person.
Insects - Studies show no evidence of HIV transmission through bloodsucking insects. This is true even in areas where there are many cases of AIDS and large populations of mosquitoes.
Using the same toilet seat
Swimming in the same pool
Touching, hugging or shaking hands
Eating in the same restaurant
Sitting next to someone

Next, we'll look at what happens once the HIV virus enters the body, and how it attacks the immune system.

HIV and Mosquitoes One of the most prevalent myths about HIV transmission is that mosquitos or other bloodsucking insects can infect you. There is no scientific evidence to support this claim. To see why mosquitoes don't aid in the transmission of HIV, we can look at the insect's biting behavior.

When mosquitoes bite someone, they do not inject its own blood or the blood of an animal or person it has bitten into the next person it bites. The mosquito does inject saliva, which acts as a lubricant so that it can feed more effectively. Yellow fever and malaria can be transmitted through the saliva, but HIV does not reproduce in insects, and therefore doesn't survive in the mosquito long enough to be transmitted in the saliva.

Additionally, mosquitoes don't normally travel from one person to another after ingesting blood. The insects need time to digest the blood meal before moving on.

To learn more about mosquitoes, check out How Mosquitoes Work.